In the soldering process, in contrast to what happens during welding, the added metal must melt, but the workpiece itself must not melt. For this reason, the melting region of the solder and the work temperature must lie beneath the melting point of the workpiece. The term "soldering" is commonly used to refer to processes for joining together metal surfaces with an alloy which may be a soft or a hard solder, and the term is used herein in this broad sense. When a hard solder is used, the process, which is generally carried out at a temperature above 450.degree. C., is also referred to and known as brazing and the hard solder is referred to as a brazing alloy. Soft or common solder is used at temperatures below 450.degree. C. and hard solder is relatively infusible as compared with soft solder.
In the soldering of aluminum and aluminum alloys, a serious problem exists by reason of the fact that a very resistant layer of oxide is present on the surface and prevents the molten solder from wetting the metal therebeneath. Although this oxide layer is very thin, it is nevertheless dense and stable, and after removal, a new film reforms spontaneously, even during the soldering process, so long as the operation is carried out in an oxidizing atmosphere such as air. It has hitherto been necessary for the soldering of aluminum-containing workpieces to remove the oxide layer from the region where the soldering is to take place, either by mechanical or by chemical means. One such method has been friction soldering, a special technique in which soft solder is applied and the oxide layer removed together with the molten solder. In a variation of this technique, ultrasonic soldering has been attempted but such technique has not come into common use.
Apart from such special processes, the removal of the oxide layer has been carried out by means of fluxes which, in addition, prevent the formation of a new oxide layer. The fluxes suitable for use in the soldering of aluminum, in general, are chlorides or fluorides and, for soft soldering, pure organic compounds are also used. All fluxes have the disadvantage that they lead to corrosion, most leading to very strong corrosion, and consequently residues must be completely removed. Moreover, there is always the danger of inclusion of flux in the solder joint. Residues and inclusions of flux damage the corrosion resistance of the solder portion, especially when dampness has access to the soldered joint. Removal of solder residues is costly and consequently is generally not complete. It is for such reasons that research has been directed to solders and especially hard solders which can operate without fluxes.
The difficulties described above become especially significant when the standards of strength and corrosion resistance for a soldered joint are considered. Solders previously used for soldering aluminum without flux have consequently not matured to actual practice or commercial production. As examples of processes that have been proposed in United States patents and publications, it has been recommended that the oxide layer be removed either through exothermic reaction or by reduction at the location where the joint is to be made, and this be carried out either in a high vacuum at an absolute pressure of about 10.sup.-6 torr, or with a combination of a reduction process in a vacuum at an absolute pressure of about 10.sup.-4 torr. In such processes, the solder, as well as any additional components, are generally added in the form of a mixture of powder.
In addition to the very dubious efficiency of the removal of oxide by the foregoing methods, these methods also have the disadvantages that the powders must be protected from oxidation during their preparation and stratification of the components must be prevented when the powder is applied to the workpiece, making such processes unsuitable for quantity production of aluminum workpieces. Also, large-scale use of high vacuum for soldering requires high costs for preparation and operation. Consequently, such processes can be used in only very special cases, such as for the manufacture of parts to be used with reactors or rockets.
As a general rule, the solders used for soldering aluminum-containing workpieces have the disadvantage of relatively high viscosity and surface tension, as well as poor wetting characteristics. This is particularly true for hard solders which, as stated above, have special significance. Such hard solders are usually of the aluminum-silicon (Al-Sl) type which may also contain copper (Cu), magnesium (Mg), nickel (Ni), zinc (Zn), tin (Sn) and cadmium (Cd). In addition to the commercial soft solders, there are zinc-aluminum (Zn-Al) solders which, according to the particular compositions and thereby the operating temperature, can be rated either as hard or as soft solders.
In order to avoid the difficulties of high viscosity and high surface tension, the flux contains in part additives in the form of zinc salts which produce a metal layer on the clean basic metal surface and increase the wetting by the solder at the point of joining.
A method for the preparation of solder to be used for soldering pure aluminum or nearly pure aluminum was disclosed in German Pat. No. 66,398, in which a substantial portion of the pure aluminum was melted and then the surface of the molten metal was covered with a layer of phosphoric acid, sodium bisulfate, fluorine compounds, or other acidic salts, and finally, to the molten metal was added a small quantity of copper and tin; or copper bismuth, zinc and tin; or copper, antimony, bismuth and zinc; or copper, bismuth, antimony and tin. Although this patent was issued in 1891, this process has never come into use. Such a solder would not be practical owing to the small difference in temperature between the melting point of the solder and of the workpiece, and the only reason why it was considered at all, was due to the fact that the melting point of aluminum was thought to be 800.degree. C. at that time (page 1, left-hand column, paragraph 2 of that patent), when it is actually 659.7.degree. C.
As was stated above, the fundamental problem in the soldering of aluminum lies in the fact that the workpiece is always covered with a skin of aluminum oxide which is only poorly wetted by the molten solder. It was therefore previously concluded that, for the achievement of satisfactory soldered joints on aluminum-containing workpieces, the oxide skin must be broken down and removed.